Particle monitors or sensors are commonly used in vacuum lines, such as exhaust lines of vacuum processing equipment for manufacturing semiconductor integrated circuits. Because many processes are run at intermediate pressures (e.g. between 0.1 to 5 Torr), the gas in an exhaust line in any such process is sufficiently dense to resist free fall of particles. Consequently, particles from the process reactor may be carried by the gas flow in the exhaust line, so that a particle sensor mounted in the exhaust line can be used to monitor such particles. An example of such particle sensor is described in copending application, Ser. No. 07/582,718, entitled "High Sensitivity, Large Detection Area Particle Sensor for Vacuum Applications," by Peter Borden et al., hereby incorporated by reference in its entirety. The particle sensor described in the above copending Application uses a laser beam to measure the rate of particles passing through a cross section of the laser beam.
FIG. 1 is a side sectional view of a vacuum line 100, in which a particle sensor 101 is mounted in a tee section 102 of a vacuum line. As shown in FIG. 1, tee section 102 is placed in line with the exhaust flow, which enters tee section 102 in a direction A at leg 102a and exits tee section 102 at leg 102b. Particle sensor 101 is plugged into the third leg 102c of tee section 102. Each vacuum connection in vacuum line 100 is made using the combination of an O-ring and a clamp ring, such as O-ring 103 and clamp ring 104. O-ring 103 is held in place by a centering ring (not shown), which is a metal ring hugging the inner circumference of the O-ring 103. Flanges, such as flange 105 in leg 102c, are provided so that the O-ring and the clamp ring can form a tight seal at the vacuum connection point.
FIG. 2 is a view of tee section 102 looking into the gas inlet (i.e. leg 102a) in direction A of gas flow. As shown in FIG. 2, particle sensor 101 has an opening 106 exposing a portion of sensing laser beam 110 projecting in a substantially parallel manner to opening 106, and orthogonal to the direction of gas flow. A portion of the exhaust gas, including the particles carried therein passes through opening 106 and sensing laser beam 110. Sensing laser beam 110 counts the particles in the portion of exhaust gas passing through it. As can be readily seen in FIG. 2, a large surface area of particle sensor 101, including its optics, is bathed in the process gas flow.
Alternatively, it is possible to place particle sensor 101 and its optics external to the pipe, and provide in vacuum line 100 windows ("sensor windows") through which sensing laser beam 110 can pass to allow detection of particles in the gas flow. In this configuration, these windows are fully exposed to the effluent gasses in the exhaust line.
Since the particle sensor is placed at a location where the process is not perturbed by the particle monitor's preference, monitoring particles using a particle sensor in the exhaust line, as provided by one of the configurations above, is an attractive technique. However, a particle sensor placed in such location may experience problems from coating, temperature and low count rate.
Specifically, many component gasses of a process effluent are species that will deposit on the exposed surface or surfaces of the particle sensor, or on the sensor windows. For example, in a chemical vapor deposition (CVD) process, the effluent gas flow contains the specie (e.g. silicon dioxide or silicon nitride) sought to be deposited on the target in the process chamber. As the effluent passses by the particle sensor in the vacuum line, the specie in the effluent gas will also be deposited on the exposed portions of the particle sensor. In addition, reaction byproducts may also be deposited. For example, in a tungsten process, byproduct tungsten oxy-fluoride compounds carried by the effluent may be deposited on the same exposed surfaces of the particle sensor. Naturally, such depositions may adversely affect sensor performance.
Further, the exhaust gasses in a vacuum line are often at elevated temperatures. For example, a CVD process may operate at 400.degree.-800.degree. C. Thus, although the temperature of the effluent gasses may have been lowered by the time the particle sensor is reached, bathing the particle sensor or the sensor windows in relatively hot gasses over a period of time may affect sensor performance adversely.
Since a particle sensor is often mounted in a vacuum line of fairly large diameter (e.g. 1.5 to 2 inches), the flow of gas and particles is fairly evenly distributed over the cross section area of the vacuum line. Consequently, in a particle counter, such as the laser beam-based particle counter mentioned above, a relatively low fraction of the particles actually go through the sensing laser beam, resulting in a low particle count. Particle count accuracy can be enhanced if more particles are channeled through the laser beam.